X-ray diffraction reconstruction of the inverted hexagonal (HII) phase in lipid-water systems

Optical and molecular features of negatively curved surfaces created by POPE lipids: A crucial role of the initial conditions

Membrane fusion is closely related to plasma membrane domains rich in cone-shaped phosphatidylethanolamine (PE) lipids that can reverse membrane curvature under certain conditions. The phase transition of PE-based lipid membranes from the lamellar fluid phase (Lα) to the inverse hexagonal phase (HII) is commonly taken as a general model in reconstructing the membrane fusion pathway, and whose structural features have been mostly described so far using structural and microscopic techniques. The aim of this paper is to decipher the optical and molecular features of Lβ → Lα and especially of Lα → HII transition of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) lipids at pH = 7.0 when they are initially prepared in the form of both multi- and unilamellar liposomes (MLVs and LUVs). The distinction between optical properties of MLS- and LUVs-derived HII phase, provided from turbidity-sensitive temperature-dependent UV-Vis spectra, was attributed to different formation mechanisms of HII phase. Most importantly, from FTIR spectroscopic data of POPE lipids in Lβ (15 °C), Lα (50 °C) and HII (85 °C) phases we identified the changes in molecular features of POPE lipids during phase transitions. Among the latter, by far the most significant is different hydration pattern of POPE lipids in MLVs- and LUVs-derived HII phase which extends from the polar-apolar interface all the way to the terminal amino group of the POPE lipid, along with the changes in the conformation of glycerol backbone as evidenced by the signature of α-methylene groups. Molecular dynamics simulations confirmed higher water penetration in HII phase and provided insight into hydrogen bonding patterns.

From angular to round: in depth interfacial analysis of binary phosphatidylethanolamine mixtures in the inverse hexagonal phase

Packing stress in the lipidic inverse hexagonal HII phase arises from the necessity of the ideally cylinder-shaped micelles to fill out the hexagonally-shaped Wigner-Seitz unit cell. Thus, hydrocarbon chains stretch towards the corners and compress in the direction of the flat side of the hexagonal unit cell. Additionally, the lipid/water interface deviates from being perfectly circular. To study this packing frustration in greater detail, we have doped 1-palmitoyl-2-oleoyl-sn-phosphatidylethanolamine (POPE) with increasing molar concentrations of 1,2-palmitoyl-sn-phosphatidylethanolamine (DPPE: 0 to 15 mol%). Due to its effectively longer hydrophobic tails, DPPE tends to aggregate in the corner regions of the unit cell, and thus, increases the circularity of the lipid/water interface. From small angle X-ray diffraction (SAXD) we determined electron density maps. Using those, we analysed the size, shape and homogeneity of the lipid/water interface as well as that of the methyl trough region. At 6 and 9 mol% DPPE the nanotubular water core most closely resembles a circle; further to this, in comparison to its neighbouring concentrations, the 9 mol% DPPE sample has the smallest water core area and smallest number of lipids per circumference, best alleviating the packing stress. Finally, a three-water layer model was applied, discerning headgroup, perturbed and free water, demonstrating that the hexagonal phase is most stable in the direction of the flat faces (compression zones) and least stable towards the vertices of the unit cell (decompression zones).

A multiscale tissue assessment in a rat model of mild traumatic brain injury

Diffusion tensor imaging (DTI) has demonstrated the potential to assess the pathophysiology of mild traumatic brain injury (mTBI) but correlations of DTI findings and pathological changes in mTBI are unclear. We evaluated the potential of ex vivo DTI to detect tissue damage in a mild mTBI rat model by exploiting multiscale imaging methods, histology and scanning micro-X-ray diffraction (SμXRD) 35 days after sham-operation (n = 2) or mTBI (n = 3). There were changes in DTI parameters rostral to the injury site. When examined by histology and SμXRD, there was evidence of axonal damage, reduced myelin density, gliosis, and ultrastructural alterations in myelin that were ongoing at the experimental time point of 35 days postinjury. We assessed the relationship between the 3 imaging modalities by multiple linear regression analysis. In this analysis, DTI and histological parameters were moderately related, whereas SμXRD parameters correlated weakly with DTI and histology. These findings suggest that while DTI appears to distinguish tissue changes at the microstructural level related to the loss of myelinated axons and gliosis, its ability to visualize alterations in myelin ultrastructure is limited. The use of several imaging techniques represents a novel approach to reveal tissue damage and provides new insights into mTBI detection.

Computational Insights into the Role of Cholesterol in Inverted Hexagonal Phase Stabilization and Endosomal Drug Release

Cholesterol is a major component of many lipid-based drug delivery systems, including cationic lipid nanoparticles. Despite its critical role in the drug release stage, the underlying molecular mechanism by which cholesterol assists in endosomal escape remains unclear. An efficient drug release from the endosome requires endosomal disruption. This disruption is believed to involve a lamellar-to-inverted hexagonal (L_α-H_II) phase transition upon fusion of the lipid nanoparticle with the endosomal membrane. We used molecular dynamics simulations to study the structural properties of H_II systems composed of an anionic lipid distearoyl phosphatidylserine (DSPS), an ionizable cationic lipid (KC2H), and cholesterol for several hydration levels and molar ratios. This system corresponds to the lipid mixtures in the hypothesized H_II structure formed upon fusion and is of interest for the rational design of ionizable cationic lipids, including KC2, for an optimal drug release. Simulations suggest a geometry- and symmetry-driven lipid sorting and cholesterol-DSPS co-location around the water cores. Cholesterol preferentially co-locates with negatively charged saturated DSPS lipids at interstitial angles. The observed cholesterol-DSPS co-location results in an overall increase in the DSPS acyl chains' order parameters, which we propose to assist in stabilizing the H_II phase by stretching the DSPS acyl chains for filling the voids formed by three adjacent lipid tubules. Furthermore, a systematic increase in the cholesterol concentration increased the lattice plane spacing and the water core radius but decreased the undulations along the lipid tubule axis. We propose that cholesterol and the degree of saturation/polyunsaturation of the lipid acyl chains, and not the lipid charge, are the main contributors in facilitating the L_α-H_II phase transition and stabilizing/destabilizing the formed H_II phase, whereas the positive charge of the ionizable cationic lipid promotes the LNP-endosomal membrane adhesion and assists in initiating the fusion process at the local contact area. We also propose that the effect of cholesterol on the H_II structure and curvature is the main underlying reason for the well-documented H_II stabilization and destabilization at low and high molar concentrations of cholesterol, respectively.

Hydrophilic nanoparticles that kill bacteria while sparing mammalian cells reveal the antibiotic role of nanostructures

To dissect the antibiotic role of nanostructures from chemical moieties belligerent to both bacterial and mammalian cells, here we show the antimicrobial activity and cytotoxicity of nanoparticle-pinched polymer brushes (NPPBs) consisting of chemically inert silica nanospheres of systematically varied diameters covalently grafted with hydrophilic polymer brushes that are non-toxic and non-bactericidal. Assembly of the hydrophilic polymers into nanostructured NPPBs doesn't alter their amicability with mammalian cells, but it incurs a transformation of their antimicrobial potential against bacteria, including clinical multidrug-resistant strains, that depends critically on the nanoparticle sizes. The acquired antimicrobial potency intensifies with small nanoparticles but subsides quickly with large ones. We identify a threshold size (d_silica ~ 50 nm) only beneath which NPPBs remodel bacteria-mimicking membrane into 2D columnar phase, the epitome of membrane pore formation. This study illuminates nanoengineering as a viable approach to develop nanoantibiotics that kill bacteria upon contact yet remain nontoxic when engulfed by mammalian cells.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. II. Structure and Functions

In Part I, by using 31P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (HII) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery. Electrochromic absorbance transients showed that the TM fragments were re-sealed, and the vesicles largely retained their impermeabilities after lipase treatments-in line with the low susceptibility of the bilayer against the same treatment, as reflected by our 31P-NMR spectroscopy. Signatures of HII-phase could not be discerned with small-angle X-ray scattering-but traces of HII structures, without long-range order, were found by freeze-fracture electron microscopy (FF-EM) and cryo-electron tomography (CET). EM and CET images also revealed the presence of small vesicles and fusion of membrane particles, which might account for one of the isotropic phases. Interaction of VDE (violaxanthin de-epoxidase, detected by Western blot technique in both membrane fragments) with TM lipids might account for the other isotropic phase. In general, non-bilayer lipids are proposed to play role in the self-assembly of the highly organized yet dynamic TM network in chloroplasts.

Design of a fluorescent and clickable Ag38(SRN3)24 nanocluster platform: synthesis, modeling and self-assembling

Fluorescent atomically precise Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters are easily prepared using sodium ascorbate as a "green" reducer and are extensively characterized by way of elemental analyses, ATR-FTIR, XRD, SAXS, UV-vis, fluorescence spectroscopies, and theoretical modeling. The fluorescence and the atomically determined stoichiometry and structure, the facile and environmentally green synthesis, together with the novel presence of terminal azido groups in the ligands which opens the way to "click"-binding a wide set of molecular species, make Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters uniquely appealing systems for biosensing, recognition and functionalization in biomedicine applications and in catalysis.

Lipid Phase Separation Induced by the Apolar Polyisoprenoid Squalane Demonstrates Its Role in Membrane Domain Formation in Archaeal Membranes

Archaea synthesize methyl-branched, ether phospholipids, which confer the archaeal membrane exceptional physicochemical properties. A novel membrane organization was proposed recently to explain the thermal and high pressure tolerance of the polyextremophilic archaeon Thermococcus barophilus. According to this theoretical model, apolar molecules could populate the midplane of the bilayer and could alter the physicochemical properties of the membrane, among which is the possibility to form membrane domains. We tested this hypothesis using neutron diffraction on a model archaeal membrane composed of two archaeal diether lipids with phosphocholine and phosphoethanolamine headgroups in the presence of the apolar polyisoprenoid squalane. We show that squalane is inserted in the midplane at a maximal concentration between 5 and 10 mol % and that squalane can modify the lateral organization of the membrane and induces the coexistence of separate phases. The lateral reorganization is temperature- and squalane concentration-dependent and could be due to the release of lipid chain frustration and the induction of a negative curvature in the lipids.

Structural Properties of Inverted Hexagonal Phase: A Hybrid Computational and Experimental Approach

Inverted/reverse hexagonal (H_II) phases are of special interest in several fields of research, including nanomedicine. We used molecular dynamics (MD) simulation to study H_II systems composed of dioleoylphosphatidylethanolamine (DOPE) and palmitoyloleoylphosphatidylethanolamine (POPE) at several hydration levels and temperatures. The effect of the hydration level on several H_II structural parameters, including deuterium order parameters, was investigated. We further used MD simulations to estimate the maximum hydrations of DOPE and POPE H_II lattices at several given temperatures. Finally, the effect of acyl chain unsaturation degree on the H_II structure was studied via comparing the DOPE with POPE H_II systems. In addition to MD simulations, we used deuterium nuclear magnetic resonance (²H NMR) and small-angle X-ray scattering (SAXS) experiments to measure the DOPE acyl chain order parameters, lattice plane distances, and the water core radius in H_II phase DOPE samples at several temperatures in the presence of excess water. Structural parameters calculated from MD simulations are in excellent agreement with the experimental data. Dehydration decreases the radius of the water core. An increase in hydration level slightly increased the deuterium order parameter of lipids acyl chains, whereas an increase in temperature decreased it. Lipid cylinders undulated along the cylinder axis as a function of hydration level. The maximum hydration levels of PE H_II phases at different temperatures were successfully predicted by MD simulations based on a single experimental measurement for the lattice plane distance in the presence of excess water. An increase in temperature decreases the maximum hydration and consequently the radius of the water core and lattice plane distances. Finally, DOPE formed H_II structures with a higher curvature compared to POPE, as expected. We propose a general protocol for constructing computational H_II systems that correspond to the experimental systems. This protocol could be used to study H_II systems composed of molecules other than the PE systems used here and to improve and validate force field parameters by using the target data in the H_II phase.

Vitamin E Promotes the Inverse Hexagonal Phase via a Novel Mechanism: Implications for Antioxidant Role

Vitamin E (α-tocopherol) and a range of other biological compounds have long been known to promote the H_II (inverted hexagonal) phase in lipids. Now, it has been well established that purely hydrophobic lipids such as dodecane promote the H_II phase by relieving extensive packing stress. They do so by residing deep within the hydrocarbon core. However, we argue from X-ray diffraction data obtained with 1-palmitoyl-2-oleoylphosphatidylcholine (POPE) and 1,2-dioleoylphosphatidylcholine (DOPE) that α-tocopherol promotes the H_II phase by a different mechanism. The OH group on the chromanol moiety of α-tocopherol anchors it near the aqueous interface. This restriction combined with the relatively short length of α-tocopherol (as compared to DOPE and POPE) means that α-tocopherol promotes the H_II phase by relieving compressive packing stress. This observation offers new insight into the nature of packing stress and lipid biophysics. With the deeper understanding of packing stress offered by our results, we also explore the role that molecular structure plays in the primary function of vitamin E, which is to prevent the oxidation of polyunsaturated membrane lipids.

Environmentally Benign Nanoantibiotics with a Built-in Deactivation Switch Responsive to Natural Habitats

The massive use of antibiotics in healthcare and agriculture has led to their artificial accumulation in natural habitats, which risks the structure and function of the microbial communities in ecosystems, threatens food and water security, and accelerates the development of resistome. Ideally, antibiotics should remain fully active in clinical services while becoming deactivated rapidly once released into the environment, but none of the current antibiotics meet this criterion. Here, we show a nanoantibiotic design that epitomizes the concept of carrying a built-in "OFF" switch responsive to natural stimuli. The environmentally benign nanoantibiotics consist of cellulose backbones covalently grafted with hydrophilic polymer brushes that by themselves are antimicrobially inactive. In their nanostructured forms in services, these cellulose-based polymer molecular brushes are potent killers for both Gram-positive and Gram-negative bacteria, including clinical multidrug-resistant strains; after services and being discharged into the environment, they are shredded into antimicrobially inactive pieces by cellulases that do not exist in the human body but are abundant in natural habitats. This study illuminates a new concept of mitigating the environmental footprints of antibiotics with rationally designed nanoantibiotics that can be dismantled and disabled by bioorthogonal chemistry occurring exclusively in natural habitats.

Monolayerwise application of linear elasticity theory well describes strongly deformed lipid membranes and the effect of solvent

The theory of elasticity of lipid membranes is used widely to describe processes of cell membrane remodeling. Classically, the functional of a membrane's elastic energy is derived under assumption of small deformations; the membrane is considered as an infinitely thin film. This functional is quadratic on membrane surface curvature, with half of the splay modulus as its proportionality coefficient; it is generally applicable for small deformations only. Any validity of this functional for the regime of strong deformations should be verified experimentally. Recently, research using molecular dynamics simulations challenged the validity of this classic, linear model, i.e. the constancy of the splay modulus for strongly bent membranes. Here we demonstrate that the quadratic energy functional still can be applied for calculation of the elastic energy of strongly deformed membranes without introducing higher order terms with additional elastic moduli, but only if applied separately for each lipid monolayer. For cylindrical membranes, both classic and monolayerwise models yield equally accurate results. For cylindrical deformations we experimentally show that the elastic energy of lipid monolayers is additive: a low molecular weight solvent leads to an approximately twofold decrease in the membrane bending stiffness. Accumulation of solvent molecules in the inner monolayer of a membrane cylinder can explain these results, as the solvent partially prevents lipid molecules from splaying there. Thus, the linear theory of elasticity can be expanded through the range from weak to strong deformations-its simplicity and physical transparency describe various membrane phenomena.

Induction of non-lamellar phases in archaeal lipids at high temperature and high hydrostatic pressure by apolar polyisoprenoids

It is now well established that cell membranes are much more than a barrier that separate the cytoplasm from the outside world. Regarding membrane's lipids and their self-assembling, the system is highly complex, for example, the cell membrane needs to adopt different curvatures to be functional. This is possible thanks to the presence of non-lamellar-forming lipids, which tend to curve the membrane. Here, we present the effect of squalane, an apolar isoprenoid molecule, on an archaea-like lipid membrane. The presence of this molecule provokes negative membrane curvature and forces lipids to self-assemble under inverted cubic and inverted hexagonal phases. Such non-lamellar phases are highly stable under a broad range of external extreme conditions, e.g. temperatures and high hydrostatic pressures, confirming that such apolar lipids could be included in the architecture of membranes arising from cells living under extreme environments.

Global small-angle scattering data analysis of inverted hexagonal phases

A global small-angle scattering model for unoriented, fully hydrated, inverted hexagonal phases is provided. The model is evaluated using Bayesian probability theory to obtain reliable estimates for the structural parameters.

A global analysis model has been developed for randomly oriented, fully hydrated, inverted hexagonal (H_II) phases formed by many amphiphiles in aqueous solution, including membrane lipids. The model is based on a structure factor for hexagonally packed rods and a compositional model for the scattering length density, enabling also the analysis of positionally weakly correlated H_II phases. Bayesian probability theory was used for optimization of the adjustable parameters, which allows parameter correlations to be retrieved in much more detail than standard analysis techniques and thereby enables a realistic error analysis. The model was applied to different phosphatidylethanolamines, including previously unreported H_II data for diC14:0 and diC16:1 phosphatid­yl­ethanolamine. The extracted structural features include intrinsic lipid curvature, hydrocarbon chain length and area per lipid at the position of the neutral plane.

Flexible lipid nanomaterials studied by NMR spectroscopy

Advances in solid-state nuclear magnetic resonance spectroscopy inform the emergence of material properties from atomistic-level interactions in membrane lipid nanostructures.

Interactions of Cationic Lipids with DNA: A Structural Approach

Colloidal nucleic acid carrier systems based on cationic lipids are a promising pharmaceutical tool in the implementation of gene therapeutic strategies. This study demonstrates the complex behavior of DNA at the lipid-solvent interface facilitating structural changes of the lyotropic liquid-crystalline phases. For this study, the structural properties of six malonic acid based cationic lipids were determined using small- and wide-angle X-ray scattering (SAXS and WAXS) as well as differential scanning calorimetry (DSC). Selected lipids (lipid 3 and lipid 6) with high nucleic acid transfer activity have been investigated in detail because of the strong influence of the zwitterionic helper lipid 1,2-di(9 Z-octadecenoyl)- sn-glycero-3-phosphoethanolamine (DOPE) on the structural properties as well as of the complex formation of lipid-DNA complexes (lipoplexes). In the case of lipid 3, DNA stabilizes a metastable cubic mesophase with Im3 m symmetry and an Im3 m Q_α^c lipoplex is formed, which is rarely described for DNA lipoplexes in literature. In the case of lipid 6, a cubic mesophase with Im3 m symmetry turns into a fluid lamellar phase while mixing with DOPE and complexing DNA.

Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction centre. Man-made systems are still far from replicating the complexity of PSII, as the majority of PSII mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Ru-polypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru₄POM). The resulting [PBI]₅Ru₄POM complex shows a robust amphiphilic structure and dynamic aggregation into large two-dimensional paracrystalline domains, a redshifted light-harvesting efficiency of >40% and favourable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ > 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

A 89Zr-labeled lipoplex nanosystem for image-guided gene delivery: design, evaluation of stability and in vivo behavior

Background

With the advances in radiopharmaceutical research, the development of image-guided therapy has become a major interest. While the development of theranostic nanotherapeutics is frequently associated with cancer chemotherapy, phototherapy and radiotherapy, there is little information available on the in vivo monitoring of gene delivery systems and the application of image-guided approach in gene therapy. The goal of this work was to determine the in vivo behavior of DNA delivery nanosystems - based on cationic gemini surfactants – designed for image-guided gene therapy. We tested the feasibility of monitoring tumor accumulation of gene delivery nanoparticles by positron emission tomography.

Methods

To be able to conjugate radiotracers to the nanoparticles, a deferoxamine-modified gemini surfactant was synthesized, DNA-containing lipoplex nanoparticles were formulated, and radiolabeled with Zirconium-89 (⁸⁹Zr). The pharmacokinetics and biodistribution of ⁸⁹Zr labeled surfactant and ⁸⁹Zr labeled nanoparticles were monitored in mice by microPET/CT imaging and ex vivo gamma counting.

Results

Modification of the nanoparticles with deferoxamine did not alter their physicochemical properties. The radiolabeled nanoparticles (labeling efficiency of 95±3%) were stable in PBS and serum. The biological half-life of the ⁸⁹Zr labeled nanoparticles was significantly higher compared to ⁸⁹Zr labeled surfactant. As expected, the nanoparticles had significantly higher liver accumulation than the radiolabeled surfactant alone and lower kidney accumulation. Tumor uptake was detected at 2 hours post injection and decreased throughout the 3-day monitoring.

Conclusion

We propose that radiolabeling DNA delivery lipoplex nanosystems is a promising approach for the design and optimization of image-guided nanomedicines, especially in the context of cancer gene therapy.

Interaction between Surface Charge-Modified Gold Nanoparticles and Phospholipid Membranes

This report clarifies the interaction of surface charge-modified gold nanoparticles (AuNPs) with phospholipid membranes, which is helpful to understand the antibacterial mechanism of positive charge-modified AuNPs to Gram-negative bacteria. Although the simulated bacterial cell membranes as a whole are negatively charged, the local electrostatic repulsive interaction between the positive charge-coated AuNPs and the small-sized flexible cationic head group of dioleyl phosphatidylethanolamine molecules induces the phase transformation of the simulated bacterial cell membranes from a lamellar to an inverted hexagonal phase. Transmembrane pores with a diameter of about 3.0 nm in the inverted hexagonal structure would result in the destruction of cell membrane function. Such an interaction of positive charge-modified AuNPs with the membrane mimics provides a promising route to develop new antibacterial agents by modifying positive charges on the surface of nanoparticles.

Molecular Engineering as an Approach To Modulate Gene Delivery Efficiency of Peptide-Modified Gemini Surfactants

The unique molecular structure confers the diquaternary ammonium gemini surfactants with enhanced nucleic acid complexation ability, bottom-up design flexibility, and relatively low cytotoxicity. To capitalize on their potential as gene delivery vectors, novel structural modifications should be explored. In this work, 22 novel peptide-modified gemini surfactants with various alkyl tails and peptide spacer modifications were evaluated. This work represents the first report of dendrimer-like gemini surfactants and first evaluation of the impact of incorporating a hydrocarbon linker into the peptide chain. Our aim was to establish a structure activity relationship of the peptide-modified gemini surfactants and to identify the fundamental architectural requirements needed for the ultimate gene delivery systems. In vitro assessment revealed that the highest transfection efficiency and lowest cytotoxicity were associated with the glycyl-lysine modified gemini surfactants having the hexadecyl tail, 16-7N(G-K)-16. In fact, it showed an 8-fold increase in secreted protein with 20% increase in cell viability relative to the first-generation unsubstituted gemini surfactants. Further increase in the size of the attached peptides resulted in a decrease in the transfection efficiency and cell viability. Whereas the incorporation of a hydrocarbon linker into the peptide chain decreased the transfection efficiency of compounds with dipeptides, it increased the transfection efficiency of compounds with larger peptide chains. Such an increase was more prominent with the incorporation of a longer hydrocarbon linker. We conclude that a balance between the hydrophilic and hydrophobic characteristics of the compound is necessary since it results in physicochemical parameters conducive to the gene delivery process.

Counterion-Dependent Access to Low-Symmetry Lyotropic Sphere Packings of Ionic Surfactant Micelles

The water-driven self-assembly of homologous dianionic surfactants into lyotropic liquid crystals (LLCs) is investigated, with a focus on understanding how surfactant headgroup and counterion identities guide supramolecular spherical mesophase selection. Using temperature-dependent small-angle X-ray scattering (SAXS), we demonstrate that 2-alkylmalonate surfactants (C_nMal-M₂) with n = 8 (octyl) or 10 (decyl) and M = K⁺, Cs⁺, or (CH₃)₄N⁺ form both simple and complex micelle packings. Observed spherical morphologies include body-centered cubic (BCC), hexagonally closest-packed (HCP), and tetrahedrally closest-packed Frank-Kasper (FK) A15 and σ phases (Pm3(-)n and P4₂/mnm symmetries, respectively). Previously observed in only one other minimally hydrated surfactant, the σ phase is a rare LLC morphology comprising a low-symmetry unit cell containing 30 sub-2-nm quasispherical micelles, each of which belongs to one of five symmetry-equivalent classes with discrete aggregation numbers. Temperature versus water concentration phase maps for C_nMal-M₂ LLCs reveal that σ-phase formation depends sensitively on the size and polarizability of the surfactant counterion and the length of the surfactant alkyl tail. These observations are rationalized in terms of a delicate interplay between global packing symmetry and local particle symmetry, and the extent to which counterion-headgroup correlations enforce the latter structures in these LLC phases.

Hydrophilic Phage-Mimicking Membrane Active Antimicrobials Reveal Nanostructure-Dependent Activity and Selectivity

The prevalent wisdom on developing membrane active antimicrobials (MAAs) is to seek a delicate, yet unquantified, cationic-hydrophobic balance. Inspired by phages that use nanostructured protein devices to invade bacteria efficiently and selectively, we study here the antibiotic role of nanostructures by designing spherical and rod-like polymer molecular brushes (PMBs) that mimic the two basic structural motifs of bacteriophages. Three model PMBs with different well-defined geometries consisting of multiple, identical copies of densely packed poly(4-vinyl-N-methylpyridine iodide) branches are synthesized by controlled/"living" polymerization, reminiscent of the viral structural motifs comprised of multiple copies of protein subunits. We show that, while the individual linear-chain polymer branch that makes up the PMBs is hydrophilic and a weak antimicrobial, amphiphilicity is not a required antibiotic trait once nanostructures come into play. The nanostructured PMBs induce an unusual topological transition of bacterial but not mammalian membranes to form pores. The sizes and shapes of the nanostructures further help define the antibiotic activity and selectivity of the PMBs against different families of bacteria. This study highlights the importance of nanostructures in the design of MAAs with high activity, low toxicity, and target specificity.

Di-Peptide-Modified Gemini Surfactants as Gene Delivery Vectors: Exploring the Role of the Alkyl Tail in Their Physicochemical Behavior and Biological Activity

The aim of this work was to elucidate the structure-activity relationship of new peptide-modified gemini surfactant-based carriers. Glycyl-lysine modified gemini surfactants that differ in the length and degree of unsaturation of their alkyl tail were used to engineer DNA nano-assemblies. To probe the optimal nitrogen to phosphate (N/P) ratio in the presence of helper lipid, in vitro gene expression and cell toxicity measurements were carried out. Characterization of the nano-assemblies was accomplished by measuring the particle size and surface charge. Morphological characteristics and lipid organization were studied by small angle X-ray scattering technique. Lipid monolayers were studied using a Langmuir-Blodgett trough. The highest activity of glycyl-lysine modified gemini surfactants was observed with the 16-carbon tail compound at 2.5 N/P ratio, showing a 5- to 10-fold increase in the level of reporter protein compared to the 12 and 18:1 carbon tail compounds. This ratio is significantly lower compared to the previously studied gemini surfactants with alkyl or amino- spacers. In addition, the 16-carbon tail compound exhibited the highest cell viability (85%). This high efficiency is attributed to the lowest critical micelle concentration of the 16-tail gemini surfactant and a balanced packing of the nanoparticles by mixing a saturated and unsaturated lipid together. At the optimal N/P ratio, all nanoparticles exhibited an inverted hexagonal lipid assembly. The results show that the length and nature of the tail of the gemini surfactants play an important role in determining the transgene efficiency of the delivery system. We demonstrated here that the interplay between the headgroup and the nature of tail is specific to each series, thus in the process of rational design, the contribution of the latter should be assessed in the appropriate context.

Small Angle X-ray Scattering for Nanoparticle Research

X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle's size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research where X-ray scattering has played a pivotal role, including in situ nanoparticle synthesis, nanoparticle assembly, and operando studies of catalysts and energy storage materials. Throughout this review we highlight the unique capabilities of X-ray scattering for structural characterization of materials in their native environment.

Small-angle scattering of particle assemblies

Small-angle scattering formulae for crystalline assemblies of arbitrary particles are derived from powder diffraction theory using the decoupling approximation. To do so, the pseudo-lattice factor is defined, and methods to overcome the limitations of the decoupling approximation are investigated. Further, approximated equations are suggested for the diffuse scattering from various defects of the first kind due to non-ideal particles, including size polydispersity, orientational disorder and positional fluctuation about their ideal positions. Calculated curves using the formalism developed herein are compared with numerical simulations computed without any approximation. For a finite-sized assembly, the scattering from the whole domain of the assembly must also be included, and this is derived using the correlation function approach.

Small-Anion Selective Transmembrane "Holes" Induced by an Antimicrobial Peptide Too Short to Span Membranes

Whereas many membrane-destabilization modes have been suggested for membrane-spanning antimicrobial peptides (AMPs), few are available for those too short to span membrane thickness. Here we show that ORB-1, a 15-residue disulfide-bridged AMP that is only ∼20 Å long even when fully stretched like a hairpin, may act by inducing small anion-selective transmembrane "holes" of negative mean curvature. In model membranes of Gram-negative bacteria, ORB-1 induces chloride transmembrane transport and formation of transmembrane channels of negative mean curvature, whereas the inactive analogue, ORB-N, does not, suggesting a correlation between antibacterial activity and ability to induce transmembrane channels. Given that ORB-N is the C-terminus amidated form of ORB-1, our results further suggest that formation of membrane-spanning dimers may be required to initiate the observed channel induction. Moreover, ORB-1 renders model bacterial membranes permeable to anions with effective hydration diameters of <1 nm (e.g., Cl(-) and NO3(-)), but not cations of similar sizes (e.g., H3O(+)), indicative of anion-selective transmembrane channels with an effective inner diameter of ≤1 nm. In addition, negative-intrinsic-curvature (NIC) lipids such as phosphoethanolamine (PE) may facilitate the membrane-destabilization process of ORB-1. Our findings may expand current understandings on how AMPs destabilize membranes and facilitate the pharmaceutical development of ORB-1.

Differential dependencies on [Ca2+] and temperature of the monolayer spontaneous curvatures of DOPE, DOPA and cardiolipin: effects of modulating the strength of the inter-headgroup repulsion

Biomembranes assume nonlamellar structures in many cellular events, with the tendency of forming a nonlamellar structure quantified by the monolayer spontaneous curvature, C(0), and with many of these events involving the acts of Ca(2+). Despite this biologically important intimacy, how C(0) is affected by [Ca(2+)] is unknown. In this study, we use the X-ray diffraction technique and the reconstruction of electron density profiles to measure the C(0)s of a zwitterionic phospholipid, DOPE, and two anionic phospholipids, DOPA and 18 : 1 (9Z) cardiolipin, at temperatures from 20 °C to 40 °C and [Ca(2+)]s from 0 mM to 100 mM; these phospholipids are chosen to examine the contributions of the electric charge density per molecule. While showing a strong dependence on temperature, C(0,DOPE) is nearly independent of [Ca(2+)]. In contrast, C(0,DOPA) and C(0),cardiolipin are almost unresponsive to the temperature change but affected by the [Ca(2+)] variation; and C(0,DOPA) varies with [Ca(2+)] ∼1.5 times more strongly than C(0,cardiolipin), with the phase preferences of DOPA and cardiolipin shifting to the H(II) phase and remaining on the Lα phase, respectively, at [Ca(2+)] = 100 mM. From these observations, we reveal the effects of modulating the strength of the inter-headgroup repulsion and discuss the mechanisms underlying the phase behaviour and cellular functions of the investigated phospholipids. Most importantly, this study recognizes that the headgroup charge density is dominant in dictating the phase behaviour of the anionic phospholipids, and that the unique molecular characteristics of cardiolipin are critically needed both for maintaining the structural integrity of cardiolipin-rich biomembranes and for fulfilling the biological roles of the phospholipid.

Phase behavior of the DOPE + DOPC + alkanol system

Small- and wide-angle X-ray diffraction was used to study the effect of 1-alkanols, as simple models of general anesthetics, (abbreviation CnOH, n = 8-18 is the even number of carbons in the aliphatic chain) on the lamellar to hexagonal Lα→ H(II) phase transition in the dioleoylphosphatidylethanolamine-dioleoylphosphatidylcholine = 3 : 1 mol/mol (DOPE + DOPC) system. All studied CnOHs were found to decrease the phase transition temperature of the DOPE + DOPC system in a CnOH chain length and concentration dependent manner and thus promote the formation of the HII phase. Anesthetically active C8OH and C10OH were found to decrease the lattice parameter d of the Lα phase, however longer non-anesthetic CnOHs increased the parameter d; this effect being more pronounced with increasing CnOH concentration. The lattice parameter of the HII phase was decreased in the presence of all CnOHs, even at the lowest concentrations studied. In the scope of the indirect mechanism of general anesthesia observed changes in the lattice parameter d (reflecting changes in the bilayer thickness) due to the intercalation of C8OH and C10OH might induce changes in the activity of integral membrane proteins engaged in neuronal pathways.

Unit cell structure of water-filled monoolein into inverted hexagonal (H(II)) mesophase modeled by molecular dynamics

The study investigates the unit cell structure of inverted hexagonal (H(II)) mesophase composed of monoolein (1-monoolein, GMO) and water using atomistic molecular dynamics methods without imposing any restraints on lipid and water molecules. Statistically meaningful and very contrast images of the radial mass density distribution, scrutinizing also the separate components water, monoolein, the polar headgroups of the lipids, the double bond, and the termini of the hydrocarbon chain (the tail), are obtained. The lipid/water interface structure is analyzed based on the obtained water density distribution, on the estimated number of hydrogen bonds per monoolein headgroup, and on the headgroup-water radial distribution functions. The headgroup mass density distribution demonstrates hexagonal shape of the monoolein/water interface that is well-defined at higher water/monoolein ratios. Water interacts with the headgroups by forming a three-layer diffusive mass density distribution, and each layer's shape is close to hexagonal, which is an indication of long-range structural interactions. It is found that the monoolein headgroups form a constant number of hydrogen bonds leaving an excessive amount of water molecules outside the first lipid coordination sphere. Furthermore, the quantity of water at the monoolein/water interface increases steadily upon extension of the unit cell, so the interface should have a very dynamic structure. Investigation of the hydrocarbon residues reveals high compression and well-expressed structuring of the tails. The tails form a very compressed and constrained structure of defined layers across the unit cell with properties corresponding to a more densely packed nonpolar liquid (oil). Due to the hexagonal shape the 2D packing frustration is constant and does not depend on the water content. All reported structural features are based on averaging of the atomic coordinates over the time-length of the simulation trajectories. That kind of processing allows the observation of the water/GMO interface shape and its stability and mobility at a time scale close to the ones of the intermolecular interactions.

Monolayer spontaneous curvature of raft-forming membrane lipids

Monolayer spontaneous curvatures for cholesterol, DOPE, POPE, DOPC, DPPC, DSPC, POPC, SOPC, and egg sphingomyelin were obtained using small-angle X-ray scattering (SAXS) on inverted hexagonal phases (HII). Spontaneous curvatures of bilayer forming lipids were estimated by adding controlled amounts to a HII forming template following previously established protocols. Spontaneous curvatures of both phosphatidylethanolamines and cholesterol were found to be at least a factor of two more negative than those of phosphatidylcholines, whose J0 values are closer to zero. Interestingly, a significant positive J0 value was retrieved for DPPC. We further determined the temperature dependence of the spontaneous curvatures J0(T) in the range from 15 to 55 °C, resulting in a quite narrow distribution of -1 to -3 × 10-3 (nm °C)-1 for most investigated lipids. The data allowed us to estimate the monolayer spontaneous curvatures of ternary lipid mixtures showing liquid ordered/liquid disordered phase coexistence. We report spontaneous curvature phase diagrams for DSPC/DOPC/Chol, DPPC/DOPC/Chol and SM/POPC/Chol and discuss effects on protein insertion and line tension.

Understanding the dark and light-enhanced bactericidal action of cationic conjugated polyelectrolytes and oligomers

A multiscale investigation was carried out to study the dark and light-enhanced bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and oligo-phenylene ethynylenes (OPEs). On the morphological scale, Gram-negative E. coli cells exposed to CPE and OPE compounds in the dark show damage to the cell envelope, plasma membrane, and in some cases the cytoplasm, while with UV-irradiation, E. coli sustained catastrophic damages to both the cell envelope and cytoplasm. In contrast, the Gram-positive S. epi bacteria appeared intact when exposed to CPE and OPE compounds in the dark but showed damages to the cell envelope with UV-irradiation. To better understand the molecular basis of CPE- and OPE-induced morphological changes and damages to bacteria, we investigated the effect of these compounds on model bacterial plasma membrane and bacterial proteins and plasmid DNA. Measurements of dark membrane perturbation activity of the CPEs and OPEs using model lipid membranes support a carpet or detergent-like mechanism by which the antimicrobial compounds induce membrane collapse and phase transitions. Under UV-irradiation, E. coli bacteria exposed to CPEs and OPEs showed covalent modifications and damages to both cellular protein and plasmid DNA, likely through oxidative pathways mediated by singlet oxygen and subsequent reactive oxygen species sensitized by the CPE and OPE compounds. Our finding thus show that the antimicrobial polymers and oligomers exert toxicity toward Gram-negative bacteria by disrupting the morphology and structures of cell envelope and cytoplasm, including cellular components such as proteins and DNA, while exert toxicity toward Gram-positive bacteria by binding to and disrupting just the cell wall.

Transition path from two apposed membranes to a stalk obtained by a combination of particle simulations and string method

The formation of an hourglass-shaped passage (stalk) connecting two apposed membranes is an essential initial step in membrane fusion. The most probable transition path from two separate membranes to a stalk, i.e., the minimum free-energy path (MFEP), is constructed using a combination of particle simulations and string method. For the reversible transition path in the coarse-grained membrane model, a collective order parameter, m, can be identified as the local difference of hydrophilic and hydrophobic densities. In particle simulations, the free energy F[m] as a functional of m is not readily available. This difficulty is overcome by an equation-free approach, where the morphology and the excess free energy along the MFEP are obtained by an on-the-fly string method. The transition state is confirmed by diagonalization of order-parameter fluctuations and by the probability of reaching either stalk or bilayer morphology from different positions along the MFEP.

Thiazolidinedione insulin sensitizers alter lipid bilayer properties and voltage-dependent sodium channel function: implications for drug discovery

The thiazolidinediones (TZDs) are used in the treatment of diabetes mellitus type 2. Their canonical effects are mediated by activation of the peroxisome proliferator–activated receptor γ (PPARγ) transcription factor. In addition to effects mediated by gene activation, the TZDs cause acute, transcription-independent changes in various membrane transport processes, including glucose transport, and they alter the function of a diverse group of membrane proteins, including ion channels. The basis for these off-target effects is unknown, but the TZDs are hydrophobic/amphiphilic and adsorb to the bilayer–water interface, which will alter bilayer properties, meaning that the TZDs may alter membrane protein function by bilayer-mediated mechanisms. We therefore explored whether the TZDs alter lipid bilayer properties sufficiently to be sensed by bilayer-spanning proteins, using gramicidin A (gA) channels as probes. The TZDs altered bilayer elastic properties with potencies that did not correlate with their affinity for PPARγ. At concentrations where they altered gA channel function, they also altered the function of voltage-dependent sodium channels, producing a prepulse-dependent current inhibition and hyperpolarizing shift in the steady-state inactivation curve. The shifts in the inactivation curve produced by the TZDs and other amphiphiles can be superimposed by plotting them as a function of the changes in gA channel lifetimes. The TZDs’ partition coefficients into lipid bilayers were measured using isothermal titration calorimetry. The most potent bilayer modifier, troglitazone, alters bilayer properties at clinically relevant free concentrations; the least potent bilayer modifiers, pioglitazone and rosiglitazone, do not. Unlike other TZDs tested, ciglitazone behaves like a hydrophobic anion and alters the gA monomer–dimer equilibrium by more than one mechanism. Our results provide a possible mechanism for some off-target effects of an important group of drugs, and underscore the importance of exploring bilayer effects of candidate drugs early in drug development.

Inorganic-organic hybrid materials through post-synthesis modification: Impact of the treatment with azides on the mesopore structure

Hybrid, hierarchically organized, monolithic silica gels, comprising periodically arranged mesopores and a cellular macroscopic network, have been prepared through a co-condensation reaction of tetrakis(2-hydroxyethyl)orthosilicate with chloromethyl-trimethoxysilane or 3-(chloropropyl)-triethoxysilane. Subsequent conversion of the chloro groups into azido groups, by nucleophilic substitution with NaN(3) in N,N-dimethylformamide, was conducted upon preservation of the monolithic structure. However, treatment with NaN(3) had a strong influence on the structure in the mesoporous regime, with changes such as an increase of mesopore diameter, pore volume and lattice constants, as well as a concomitant decrease of the pore wall thickness, as confirmed by small angle X-ray scattering, transmission electron microscopy, and nitrogen sorption analysis. Similar effects were observed for unmodified silica gels by simple ageing in azide-containing media, whether a relatively small or a sterically demanding counter ion (Na(+) or (H(3)C)(4)N(+)) was used. The structural modification did not seem to depend greatly on whether an organic aprotic solvent (N,N-dimethylformamide, 1,1,3,3-tetramethylurea, 1,3-dimethyl-2-imidazolidinone) or a protic solvent that can form hydrogen bonds, such as water, was used.

From glycosylation disorders to dolichol biosynthesis defects: a new class of metabolic diseases

Polyisoprenoid alcohols are membrane lipids that are present in every cell, conserved from archaea to higher eukaryotes. The most common form, alpha-saturated polyprenol or dolichol is present in all tissues and most organelle membranes of eukaryotic cells. Dolichol has a well defined role as a lipid carrier for the glycan precursor in the early stages of N-linked protein glycosylation, which is assembled in the endoplasmic reticulum of all eukaryotic cells. Other glycosylation processes including C- and O-mannosylation, GPI-anchor biosynthesis and O-glucosylation also depend on dolichol biosynthesis via the availability of dolichol-P-mannose and dolichol-P-glucose in the ER. The ubiquity of dolichol in cellular compartments that are not involved in glycosylation raises the possibility of additional functions independent of these protein post-translational modifications. The molecular basis of several steps involved in the synthesis and the recycling of dolichol and its derivatives is still unknown, which hampers further research into this direction. In this review, we summarize the current knowledge on structural and functional aspects of dolichol metabolites. We will describe the metabolic disorders with a defect in known steps of dolichol biosynthesis and recycling in human and discuss their pathogenic mechanisms. Exploration of the developmental, cellular and biochemical defects associated with these disorders will provide a better understanding of the functions of this lipid class in human.

Elastic deformations in hexagonal phases studied by small-angle X-ray diffraction and simulations

In this study we present experimental and theoretical results which concern the deviations from circularity of the pivotal plane in the inverse hexagonal phases (H(II)) of phospholipid self-assemblies. Due to packing constraints, the cross-section of the polar/apolar interface deviates from a circle, which we studied in minute detail by analysing small-angle X-ray diffraction data of dioleoyl-phosphatidylethanolamine (DOPE) and stearoyl-oleoyl-phosphatidylethanolamine (SOPE), respectively. On this structural basis, Monte Carlo (MC) simulated annealing variations of the free energy were carried out, both on the formation of the H(II)-phase and on the particular shape of the cross-section in the H(II)-phase. The equilibrium of the H(II)-phase pivotal plane contour and the corresponding values of the mean intrinsic curvature, H(m), and the hydrocarbon chain stiffness, τ, were determined from MC calculations. The results of these calculations were tested by solving the corresponding system of non-linear differential equations derived using variational calculus. Here our main aim is to predict the range of possible values of H(m) and τ. Comparing the measured structural data with predictions from MC calculations including lipid anisotropy, and accounting for the elastic deformations of the pivotal plane allowed us to determine a relationship between the bending deformation and stretching of hydrocarbon chains.

The multiple faces of self-assembled lipidic systems

Lipids, the building blocks of cells, common to every living organisms, have the propensity to self-assemble into well-defined structures over short and long-range spatial scales. The driving forces have their roots mainly in the hydrophobic effect and electrostatic interactions. Membranes in lamellar phase are ubiquitous in cellular compartments and can phase-separate upon mixing lipids in different liquid-crystalline states. Hexagonal phases and especially cubic phases can be synthesized and observed in vivo as well. Membrane often closes up into a vesicle whose shape is determined by the interplay of curvature, area difference elasticity and line tension energies, and can adopt the form of a sphere, a tube, a prolate, a starfish and many more. Complexes made of lipids and polyelectrolytes or inorganic materials exhibit a rich diversity of structural morphologies due to additional interactions which become increasingly hard to track without the aid of suitable computer models. From the plasma membrane of archaebacteria to gene delivery, self-assembled lipidic systems have left their mark in cell biology and nanobiotechnology; however, the underlying physics is yet to be fully unraveled.PACS Codes: 87.14.Cc, 82.70.Uv.